1. Field of the Invention
This invention relates to a micro-porous sheet and, in particular, to a micro-porous sheet having improved stability at high temperatures.
2. Description of the Prior Art
Micro-porous sheets have long been known (for example, refer to R. Kesting Synthetic Polymer Membranes, McGraw-Hill (1971)) and are used widely, for example, as a filter.
Micro-porous sheets have heretofore been produced starting from cellulose esters as described, for example, in U.S. Pat. Nos. 1,421,341, 3,133,132 and 2,944,017 and Japanese Pat. Publication Nos. 15,698/68, 33,313/70, 39,586/73 and 40,050/73. However, the micro-porous sheets produced from cellulose esters have limited applications because they are very unstable at high temperatures. For example, micro-porous sheets containing cellulose acetate as a basic component and used for the filtration of pharmaceuticals have a disadvantage in that the filtration capacity of the sheets is markedly reduced, because of the hydrolysis of the cellulose acetate, through heating with steam under pressure or the like, which conditions are usually employed in the sterilization of the sheet before use. Therefore, only specific and restricted methods such as gas sterilization can be employed for the sterilization of the micro-porous sheets containing cellulose acetate as a basic component, so that the applications of such mirco-porous sheets have been limited markedly in the pharmaceutical industry.